1. Field of the Invention
This invention relates to a front-diaphragm lens, and particularly to a front-diaphragm wide angle lens having an angle of view from 65.degree. to 70.degree. while being very simple in construction.
2. Description of the Prior Art
In small type cameras, it is often desirable that a diaphragm is located outside the lens because of the simplification of constructio of the camera.
A front-diaphragm lens wherein a diaphragm is arranged forwardly of a lens has an advantage that may utilize an image space between a lens and an image surface, and therefore, it is advantageous to arrange constituent members of a small camera. In addition, since an entrance pupil is free from deformation caused by an optical system, 100% of relative illumination may be obtained unless eclipse caused by lens is present. Thus, the front-diaphragm lens is particularly useful for the wide angle lens.
In this case, however, the lens is arranged extremely asymmetrically relative to the diaphragm, and therefore, compensation of comatic aberration and astigmatism is difficult to make and this tendency is further remarkable to provide a wide angle. It has been said that in a lens of simple construction comprising about three lenses, an angle of view exceeding 50.degree. is impossible to be employed. For example, a lens of this kind is known from Japanese Patent Application Laid-Open No. 36226/78. This lens has a large aperture ratio of 1:3.5 but has an angle of view of 46.degree., and at the maximum, only 50.degree.. If a half angle of view exceeds 15.degree., meridional astigmatism results in a surplus compensation, and therefore, the lens is not possible to be used as a wide angle lens.